1. Field of the Invention
The present invention relates to NICAM (Near Instantaneous Compound Audio Multiplex) broadcasting and, more particularly, to a digital audio broadcasting receiving apparatus having a capability of automatically switching between a NICAM audio signal and an analog audio signal by means of the information represented by a control bit contained in a NICAM signal.
2. Description of Related Art
NICAM broadcasting, one of several television audio multiplexing schemes, is for multiplexing digital audio signals to be transmitted. Especially, Europe has started to provide the service of NICAM broadcasting in addition to the conventional teletext. It is therefore required in Europe that the CATV (Cable Television) has the multichannel receiving capability and, at the same time, can decode the digital audio signal of NICAM broadcasting with good reproducibility. The European NICAM broadcasting carries data at a transfer rate of 728 kbits/second. The data includes a frame, two audio channels (704 kbits), and indication data (11 kbits). NICAM broadcasting therefore can transmit up to three channels of different audio information including analog audio (one channel).
Thus, NICAM broadcasting provides service with analog audio and digital audio combined. A digital broadcast receiving apparatus that receives NICAM broadcasting needs to determine which audio signal is being received to decode the received audio signal with good reproducibility. To allow the receiving apparatus to make the determination, the NICAM signal contains control bits for indicating in which state a particular transmission is being made. The control bits include C0, C1, C2, C3, and C4. The control bit C4, a reserve audio switching flag, is assigned as an indication bit for indicating a relationship between contents of analog audio and NICAM audio. Consequently, with the conventional digital audio broadcast receiving apparatus, it is determined whether to automatically switch between NICAM audio and analog audio based on the information represented by the reserve audio switching flag C4. U.S. Pat. No. 5,138,457 is art related to the present application.
Turning to the above-mentioned digital audio broadcast receiving apparatus, the automatic switching is made also in considering a case in which NICAM audio and analog audio represent two different programs, thereby making the switching too late if the NICAM audio and analog audio represent the same program. Since NICAM audio is based on PCM (Pulse Code Modulation), switching that is too late increases an error rate, thereby impairing listenability. To solve this problem, assume that the switching be set to an earlier point. Such a setting, however, causes analog audio to be automatically selected when a user wants NICAM audio even if its sound quality is less desirable when NICAM audio and analog audio represent separate programs. Each time the automatic selection occurs, the user must make audio switching manually. In addition, some receiving apparatuses permit no manual switching.